The present invention relates to a selenium electrophotographic laminate photoreceptor which is used in ordinary copying machines and optical printers having light-emitting diodes, laser diodes, gas lasers, or the like as light sources.
In making hard copies, optical printers have been widely used owing to high copying speeds and good image quality. The wavelength of light from the light source of such an optical printer is in the range of 660 to 800 nm, namely, a long wavelength range. An electrophotographic photoreceptor with a charge generating layer made of a high-concentration Te-Se alloy containing 20 to 50 wt % of tellurium so as to have excellent electrophotographic properties in a long wavelength range is disclosed in applicants' Japanese Patent Laid-Open No. 278858/1986, and has been put to practical use.
The printing durability of such a photoreceptor having a carrier generating layer of a high-concentration Te-Se alloy is determined by the overcoat layer. It is known that in order to enhance the printing durability, a selenium arsenic alloy with an increased arsenic concentration is used for the overcoat layer. However, when the arsenic concentration is increased, the thermal expansion coefficient of the overcoat layer is reduced, as shown in FIG. 2, so that the difference in the thermal expansion coefficient between the surface and the base layer of a Te-Se alloy or a carrier transportation layer is increased, thereby causing cracking. To prevent this, it is necessary to reduce the thickness of the overcoat layer, which decreases printing durability. To solve this problem, Japanese Patent Laid-Open No. 278858/1986 proposes that the overcoat layer have a two-layer structure and that the layer adjacent to the base layer have a lower arsenic concentration so as to serve as a buffer layer for the difference in the thermal expansion coefficient.
The object of the present invention is to enhance the effect of the above-described structure and to provide a selenium electrophotographic photoreceptor having long wavelength sensitivity, heat resistance, and improved printing durability.